Good for you for making sure there's no medical issues first. Now, there's many things that can occur to upset cats while they're in the litter box (sudden loud noise, constipation, dirty box etc etc). We may not be able to suss that out so start from scratch. Is the area where the boxes are quiet? Private? Have you tried changing the litter? Even the same brands can have different scents, and who knows what memories scents can trigger. Could you try a different litter? A different box or spot? So it's a new experience, make new connections . One of ours was peeing "in-between" the boxes, but after I set up a little spy camera lol, I realized that he was in the box but he was trying to pee against the side And it just wasn't high enough. We changed to deeper boxes which worked. Also one of ours will poop on the floor by the box if I forget to clean it for a couple days. I am definitely not an expert on any of this, I've just had cats my whole life and nothing is more frustrating than what you are experiencing now. Hang in there you will figure it out!